Activity of oxygen in liquid co-ni alloys
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t i o n , with or without i m p u r i t i e s , is c o m p l e t e d by the f o r m a t i o n of a q u a s i b i n a r y e u t e c t i c made up of a l u m i n u m solid solution and a phase which i s c r y s t a l l o g r a p h ically e q u i v a l e n t to MgZn2, as shown in the a u t h o r s ' Fig. 5(a). However, Strawbridge et a l l ~ shown that z i n c in this phase can be s u b s t i t u t e d for by a l u m i num plus copper, to the e x t r e m e of a t e r n a r y Mg(A1,Cu) phase c o n t a i n i n g no zinc at all. T h e r e f o r e , a high c o n c e n t r a t i o n of zinc, copper, and m a g n e s i u m can be expected to be p r e s e n t in e u t e c t i c r e g i o n s of cast a l loy 7075. Upon r e h e a t i n g to s u f f i c i e n t l y high t e m p e r p e r a t u r e , all t h r e e e l e m e n t s tend to dissolve but, s i m u l t a n e o u s l y , A12CuMg f o r m s v e r y rapidly from r e s i d u a l MgZn2 (containing A1 + Cu) by a solid state r e a c t i o n analogous to a p e r i t e c t i c or p e r i t e c t o i d t r a n s f o r m a t i o n . T h i s new phase, in t u r n s d i s s o l v e s at a much lower r a t e . The c o m p o s i t i o n range of alloy 7075 with r e s p e c t to m a g n e s i u m , z i n c , and copper i s such that some specific alloy c o m p o s i t i o n s at some s o l u t i o n i z i n g t e m p e r a t u r e s can have a solid solution s a t u r a t e d with these t h r e e e l e m e n t s and the e x c e s s always a p p e a r s as A12CuMg. R a i s i n g the s o l u t i o n i z i n g t e m p e r a t u r e will, of c o u r s e s c a u s e m o r e r a p i d and complete d i s s o l u t i o n . It is not u n c o m m o n to u s e t e m p e r a t u r e s a s high as 500~ for s o l u t i o n t r e a t m e n t of light gage sheet where a l e s s e r d e g r e e of h e t e r o geneity e x i s t s than in l e s s highly worked products. The e q u i l i b r i u m solidus t e m p e r a t u r e lies s t i l l higher than 500~ but local c o n c e n t r a t i o n s of A12CuMg p a r t i c l e s can r e s u l t in n o n e q u i l i b r i u m i n c i p i e n t m e l t i n g at t e m p e r a t u r e s a s low as 476~ u n l e s s the solutionizing t e m p e r a t u r e is approached v e r y slowly. T h e r e a r e o c c a s i o n s when s m a l l a m o u n t s of CuA12 appear c o m i n g l e d with MgZn2 in the a s - c a s t eutectic of alloy 7075, r e s u l t i n g f r o m a b n o r m a l l y high p r o p o r t i o n s of copper to m a g n e s i u m in the alloy. However, this CuA12 also d i s a p p e a r s r a p i d l y upon s u b s e q u e n t h e a t i n g , and it i s c l e a r l y r e p l a c e d by AlzCuMg. The m o s t i m p o r t a n t point of this d i s c u s s i o n conc e r n s the r e l i a b i l i t y of m a k i n g phase i d e n t i f i c a t i o n s by m i c r o p r o b e a n a l y s i s without b e n e f i t of other c r y s t a l l o g r a p h i c or m e t a l l o g r a p h i c e v i d e n c e . It a p p e a r s that some h a z a r d s exist and that m o r e defi
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